1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a facsimile device and a printer, and particularly to a developing device using a two-component developer and an image forming apparatus equipped with the developing device.
2. Description of the Related Art
Conventionally, a developing device that uses a two-component developer composed of a toner and a magnetic carrier is provided with, independently, a developer supply conveyance path for supplying the developer to a developing roller serving as a developer carrier, and a developer agitation conveyance path for conveying the developer, so that the developer can be circulated while being conveyed along these two conveyance paths in opposite directions. Such a developing device uses the developer supply conveyance path also as a developer recovery conveyance path for recovering the developer that has passed through a developing region after being supplied to the developing roller and has consumed its toner.
However, if the developer supply conveyance path and the developer recovery conveyance path are used as one conveyance path, a developer that has consumed its toner is mixed in with a developer that has been adjusted to obtain an appropriate toner density so as to be supplied to the developing roller. For this reason, there arises a problem that the toner density of the developer to be supplied to the developing roller is reduced, and thereby the image density at the time of development is reduced.
This problem can be solved by providing the developer supply conveyance path and the developer recovery conveyance path as two different developer conveyance paths, as in the developing device described in, for example, Japanese Published Unexamined Patent Application H11-167260. This developing device is provided with a developer supply conveyance path and a developer recovery conveyance path separately. Moreover, this developing device has a developer agitation conveyance path, which, while agitating a developer that has been conveyed to the lowermost stream side of the developer supply conveyance path in a developer conveyance direction and a recovery developer that has been conveyed to the lowermost stream side of the developer recovery conveyance path in a developer conveyance direction, conveys the developers in a direction opposite to that of the developer supply conveyance path.
However, in such a configuration of this developing device, when a latent image with a small image area ratio is developed, the amount of developer to be recovered by the developer recovery conveyance path is increased, and thereby the amount of recovery developer to be transported from the downstream side of the developer recovery conveyance path to the upstream side of the developer agitation conveyance path is increased. Therefore, when latent images with a small image area ratio are developed continuously, the amount of developer existing on the upstream side of the developer agitation conveyance path increases gradually. When the amount of developer existing on the upstream side of the developer agitation conveyance path increases, the recovery developer to be transported from the downstream side of the developer recovery conveyance path to the upstream side of the developer agitation conveyance path starts flowing slowly, whereby the recovery developer accumulates on the downstream side of the developer recovery conveyance path in the developer conveyance direction. Then, when the bulk of the recovery developer becomes excessively high at the downstream side of the developer recovery conveyance path in the developer conveyance direction due to this accumulation, the recovery developer re-adheres to a developing roller, which is so-called “accompanying phenomenon.” When this accompanying phenomenon occurs, the developer which has been used for development and thereby the toner density of which has decreased adheres to the developing roller, hence there arises a problem that the image density of only this adhered part becomes low, causing a white stripe and deteriorating the image quality.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Published Unexamined Patent Application No. 2006-243255.